Mechanisms for the NAO Responses to the North Atlantic SST Tripole

Abstract

The response of an atmospheric general circulation model (GCM) to the North Atlantic SST tripole exhibits both symmetric and asymmetric components with respect to the sign of the SST anomaly. The symmetric part of the response is characterized by a North Atlantic Oscillation (NAO)?like dipole with an equivalent barotropic structure over the Atlantic. The asymmetry is manifested in a weaker and smaller-scale dipole response to the positive SST tripole in contrast to a stronger and more zonally elongated dipole response to the negative tripole. Mechanisms for developing and maintaining these GCM responses are elucidated through diagnostic experiments using a linear baroclinic model and a statistical storm track model based on GCM intrinsic variability. The NAO-like symmetric response is primarily maintained by a dipolar anomalous eddy forcing that results from interactions between the heating-forced anomalous flow and the Atlantic storm track, as expected from an eddy-feedback mechanism. To account for the asymmetry of the responses about the sign of the SST tripole, a nonlinear eddy-feedback mechanism is proposed that extends the previous mechanism to include the nonlinear self-interaction of the heating-forced anomalous flow and its effects on transient eddy feedbacks. The results of idealized model experiments demonstrate that, due to its nonlinear self-interaction, the tripole heating induces a much weaker response in the positive phase than in the negative phase. Interactions of these nonlinear heating-forced anomalous flows with the Atlantic storm track result in asymmetric eddy vorticity forcings that in turn sustain asymmetric eddy-forced anomalous flows in the two cases.